Ring laser gyroscopes are frequently used to sense angular rates in order to guide and navigate a variety of vehicles such as airplanes, rockets, tanks, ships, submarines, drilling rigs, etc. As shown in FIG. 1, a ring laser gyroscope 10 is typically formed of a block 12 of material such as Zerodur® which has a low coefficient of thermal expansion. Accordingly, the block 12 is resistant to expansion over a wide temperature range. The block 12 is provided with interior passages 14 that communicate with openings at each of its corners. Mirrors 16, 18, and 20 are provided at the corners with one of the mirrors 16, 18, and 20 being used as a read-out device. The interior passages 14 and the mirrors 16, 18, and 20 define a plasma chamber in the form of a closed laser resonant path.
A cathode 22 and anodes 24 and 26 engage corresponding surfaces of the block 12 at openings there through. Indium is usually used to form seals between the block 12 and the electrodes comprising the cathode 22 and the anodes 24 and 26. These seals confine the gas that is energized to provide the laser plasma within the plasma chamber. The energized gas is often referred to as the discharge gas. The Indium seals are compressible so that a tight seal is formed. Also, a dither motor 28 to be discussed below is provided between the block 12 and a support structure.
A source 30 supplies an electric potential across the cathode 22 and the anodes 24 and 26. Typically, the source 30 biases the anodes 24 and 26 at or slightly negative with respect to the potential of the block 12, particularly the potential in the area of the block 12 at the dither motor 28. This potential is a reference potential such as ground. The source 30 biases the cathode 22 at a potential that is more negative than the potential of the anodes 24 and 26.
Accordingly, the electric potential across the cathode 22 and the anode 24 energizes the gas in the interior passages 14 so as to form a laser plasma that supports a laser which traverses the optical closed loop provided by the interior passages 14 in one direction such as a clockwise direction. Similarly, the electric potential across the cathode 22 and the anode 26 energizes the gas in the interior passages 14 so as to form a laser plasma that supports a laser which traverses the optical closed loop provided by the interior passages 14 in the opposite direction such as a counterclockwise direction.
One of the problems associated with the ring laser gyroscope 10 is lock-in which occurs at low rotation rates. Retroscatter from the mirrors 16, 18, and 20 within the optical path formed by the interior passages 14 couples energy from one of the lasers into the counter-propagating laser. When such coupling occurs, the oscillating frequencies of the two counter-propagating lasers lock together in a single frequency. Thus, a ring laser gyroscope can be insensitive to rotations having low rates. Accordingly, the dither motor 28 is provided in order to dither the ring laser gyroscope 10 because dithering mitigates lock-in.
Another of the problems associated with the ring laser gyroscope 10 is ionic current flow. Zerodur® is a lithium-aluminum-silicate glass ceramic material. Glass and glass ceramics are subject to ionic conductivity in which ionic currents are created through the material of the block 12 whenever an electric potential is applied across the block, such as when the cathode 22 and the anodes 24 and 26 are energized. Accordingly, positively charged alkali ions, such as lithium ions, flow as an ionic current toward the cathode 22.
The accumulation of these positively charged ions at the cathode 22 can adversely impact the performance of the ring laser gyroscope 10. For example, the lithium ions can attack the lithium seals between the cathode 22 and the block 12. Various solutions to this problem are offered in U.S. Pat. No. 5,098,189 which discloses the use of a slot and/or secondary negative electrodes to reduce or prevent lithium ion migration, in U.S. Pat. No. 5,856,995 which discloses the use of a trap electrode to attract the lithium ions, and in U.S. Pat. No. 6,025,914 which discloses the use of a dielectric barrier to reduce lithium ion migration.
However, insufficient oxygen that is available to maintain the metal oxide that coats the cathode 22 also can adversely impact the performance of the ring laser gyroscope 10 because the sputter life of the ring laser gyroscope 10 decreases as the available oxygen decreases. Oxygen depletion can result from absorption of the oxygen by positively charged ions on the surfaces of the interior passages 14 of the block 12. Also, positively charged ions at the interior passages 14 prevents the outflow of oxygen from the glass material of the block 12 into the discharge thereby reducing the oxygen available to protect the mirrors 16, 18, and 20 and the cathode 22.
The present invention is directed to arrangements which prevent the absorption and/or outflow of oxygen by positively charged ions such as lithium ions. Accordingly, the present invention increases the sputter life of a ring laser gyroscope.